1. Field of the Invention
This invention relates to the radio communication field. More particularly, this invention relates to a system which performs QoS control by varying the slot length in a system where a plurality of wireless terminals communicate with a common base station by Time Slot Sharing (TSS). One system of this type is a wireless local area network (LAN). This invention can be applied suitably to such a radio communication system.
2. Description of the Related Art
In recent years, the techniques for wireless LAN systems complying with the IEEE standard 802.11 series have been developed actively. The standard covers the technique for maintaining the channel while varying the communication quality, by changing the data rate and modulation method according to the situation of the wireless link. This type of technique is known as multi-rate control or link adaptation. Recently, the combination of these and the concept of (quality of service (QOS) has been studied (refer to IEEE Std 802.11e/D4.3, May 2003 Draft Supplement to standard for Telecommunications and Information Exchange Between System—LAN/MAN Specific Requirements—Part 11: Wireless Medium Access Control [MAC] and Physical Layer [PHY] specifications: Medium Access Control [MAC] Enhancements for Quality of Service [QoS]).
Another technique for combining link adaptation with QoS in allocating wireless resources to terminals has been disclosed (for example, refer to Jpn. Pat. Appln. KOKAI Publication 2002-78012). In this document, however, no consideration has been given to a case where QoS is not satisfied since the communication quality has degraded due to link adaptation after the setting of the channel. Moreover, in this document, the index of measuring QoS is only the priority of transmission data.
The IEEE standard 802.11e/D4.3 has prescribed that the QoS of communication data (traffic) should be evaluated on the basis of not only the priority of transmission data but also such parameters as the minimum data rate, the maximum delay bound, and the inactivity interval in a wireless LAN. Therefore, to combine link adaptation with QoS in a radio communication system complying with the standard, the wireless band has to be managed, taking into account temporal changes in the communication quality and various parameters of QoS.
As described above, in the link adaptation techniques, there is a possibility that the communication quality will vary during communication. Since in the existing ratio communication systems, these things have not been taken into account, the communication quality can deteriorate due to adaptation during communication, possibly preventing QoS from being met. In addition, although a plurality of parameters have to be evaluated comprehensively in measuring QoS, there has been no system which has taken this into account.